(1) Field of the Invention
The present invention relates to a wireless communication apparatus, and more particularly to a wireless communication apparatus including an indoor unit and an outdoor unit for use in a wireless communication of a millimeter frequency band or a quasi-millimeter frequency band.
(2) Description of Related Art
FIG. 23 is a diagram showing in a block form one example of a conventional wireless communication apparatus for use in a millimeter frequency band or a quasi-millimeter frequency band. As shown in FIG. 23, a wireless communication system 100 is arranged to include buildings of local stations 200, 300 such as a telephone switching center located in an opposing fashion. Each of the local stations 200 and 300 is arranged to include an indoor unit (hereinafter referred to as IDU) 201, 301 provided within the building and an outdoor unit (hereinafter referred to as ODU) 202, 302 provided on a steel tower 203, 303 near the building of the local station 200, 300.
In the following description, a set of IDU and ODU is referred to as a millimeter wireless communication apparatus or simply referred to as a wireless communication apparatus. Further, the term “millimeter” or “quasi-millimeter” means a frequency band of a radio wave at about 13 GHz to 38 GHz.
In this case, the above-introduced IDU 201 (301) up-converts a transmitting baseband signal into an intermediate frequency band signal (IF signal) and transmits the IF signal to the ODU 202 (302) with a cable communication so as to transmit to the opposing station 300 (200). Also, the IDU 201 (301) has a function for down-converting an IF signal received from the ODU 202 (302) to obtain a baseband signal. The ODU 202 (302) also up-converts the IF signal received from the IDU 201 (301) into an RF (radio frequency band) signal of a millimeter wave (or quasi-millimeter wave) band (a frequency of 13 GHz or more) and transmits the RF signal to the opposing station 300 (200) with a wireless communication. Also, the ODU 202 (302) has a function to receive an RF signal transmitted from the opposing station 300 (200) and down-convert the received RF signal into an IF signal to transmit to the IDU 201 (301).
In the above-described conventional wireless communication system 100, a cable communication using an IF signal is effected between the IDU 201 (301), and the ODU 202 (302) and a wireless communication using an RF signal is effected between the ODUs 202 and 302 opposing to each other.
However, the above-introduced wireless communication system 100 using millimeter wave (or quasi-millimeter wave) tends to suffer from loss between feeders or rain attenuation due to the property of an electromagnetic wave of the millimeter wave (or quasi-millimeter wave). Thus, such wireless communication system has not been suitable for a long-distance transmission and not been widely deployed very much.
However, owing to the recent explosive deployment of the internet, an access channel wireless system such as a mobile communication unit is developed rapidly and the number of carrier waves allowable to be accommodated within the C-band for long-distance transmission comes to the limit. For this reason, there can be observed a rapid increase of demand in a wireless communication in a millimeter wave or quasi-millimeter wave.
For example, in a communication network using mobile communication units, the distance between radio base stations is relatively short and hence signal transmission can be positively carried out by using a wireless transmission system using the millimeter wave. Thus, it can be considered that a millimeter wave is utilized for communication between the radio base stations. Further, the utilization of millimeter wave is not limited to the above application but, for example, it is considered that firms located with a relatively short distance between them are connected by an exclusively utilized network using a millimeter wave. Furthermore, as shown in FIG. 24, it is considered that a number of millimeter wireless communication apparatus 400 are connected in a multi-hop fashion so as to realize a long-distance transmission substantially equivalent to the transmission distance at the C-band (about 50 to 100 Km).
However, according to the conventional millimeter wireless communication apparatus, a possible transmission distance is limited by a single wave that is assigned to a single unit of ODU. Therefore, if the transmission capacity is to be increased, the possible transmission distance has to be shortened. Alternatively, as shown in FIG. 25, a whole communication system including the IDUs and ODUs shall be additionally provided.
For this reason, if the communication network using the millimeter wave is additionally provided with the above-illustrated multi-hop connection channel, each of the base stations shall be additionally provided with the IDU (channel unit) and the ODU, which fact leads to serious disadvantage in terms of cost of facility. Particularly, since parts for constructing the ODU for millimeter wave communication are very expensive, it becomes a heavy burden to additionally provide an ODU from a cost standpoint. Further, the whole communication network cannot cope with the additional installation of a system or increase and/or decrease of carrier wave number with flexibility. Thus, the network can always service only a fixed amount of transmission capacity to every end user.
In order to solve the above problem, it can be considered that a plurality of IDUs are made connectable to a single unit of ODU through a combiner-distributor so that the communication network can cope with the additional installation of carrier (channel) with flexibility. However, in this case, the transmission distance shall be made short so that a single unit of ODU can transmit a multichannel signal employing a plurality of millimeter waves as carrier waves without no trouble.
However, if the wireless communication apparatus is simply arranged such that signals (IF signals) received from a plurality of IDUs are combined and fed to the ODU, when an IDU is additionally provided for a single wave, a transmitting power is abruptly increased, with the result that a negative influence is caused on channels in service. Further, if the signals received from the plurality of IDUs are combined and the resultant combined signal is received as a multichannel signal as described above, the apparatus on the receiving side needs to separate (extract) each channel signal from the received multichannel signal using a filter or the like which corresponds to the frequency of the channel signal and transmit the extracted channel signal to the corresponding IDU. However, if the receiving process includes the filter process for separating the channel signal, the frequency (channel frequency) which each IDU can process comes to be fixed.
The above fact is extremely disadvantageous when each IDU is shared for processing a plurality of channel signals. In particular, according to the recommendation of the ITU-R (International Telecommunication Union-Radio communication sector), the channel interface of a millimeter wireless communication apparatus is provided with three kinds of modes, or 2 MHz×4, 2 MHz×8, 2 MHz×16, and a band for one wave differs for each mode. Thus, if the channel frequency which each IDU can process comes to be fixed, the communication apparatus shall be provided with an IDU for each fixed frequency for each mode.